1. Field of the Invention
The present invention relates to power factor correction (hereinafter simply referred to as "PFC" converter, and more particularly to a PFC converter provided with a booster control circuit controlled for decreasing harmonic current for constantly maintaining a range of a stable voltage of a bulk condenser to improve a power factor.
2. Description of the Prior Art
Generally, a voltage flowing through an electric transmission line is formed by periodic waveforms. Also, in respective components of the periodic waveform, harmonic current being a harmonic noise which is to be n-harmonic wave having a frequency of n times of a fundamental wave exists as well as the fundamental wave. However, recently in respective countries, when a voltage is supplied onto the electric transmission lines, the voltage is transmitted after being boosted for lowering a transmission loss. For this operation, a coil is provided to the preceding stage of a bulk condenser being the primary side of the input terminal of a transformer which forms an electric power supplying apparatus to prevent the supply of an overvoltage to the bulk condenser, or a boost-up circuit is provided to switch for chopping the voltage transmitted to be applied into the electric power supplying apparatus. By this construction, the harmonic current is lowered at the voltage supplied into the electric power supplying apparatus to enhance the power factor.
FIG. 1 is a circuit diagram showing a smoothing converter according to a conventional technique.
The conventional smoothing converter is formed by a bridge diode 10 for rectifying an input voltage Vi, and a smoothing section 12 for lowering a harmonic current in an output voltage of bridge diode 10 to improve the power factor. Also, a transformer T excites the output of smoothing section 12 from the primary coil side toward the second coil side by the switching of a switch section Q which is switched for allowing the output of smoothing section 12 to be pulsewidth-modulated under the control of a control section 16. An output section 14 rectifies and smooths the secondary side output of transformer T, and control section 16 controls switch section Q by the feedback of an output voltage Vo of output section 14.
In the smoothing converter according to the conventional technique formed as above, when input voltage Vi is applied to be higher than the potential of first and second condensers C1 and C2 connected to a second diode D2 of smoothing section 12, it charges up first condenser C1 by the forward supply of second condenser C2 and second diode D2. If input voltage Vi is applied to be lower than the potential of first and second condensers C1 and C2, the voltage charging up first and second condensers C1 and C2 is applied to transformer T by the operation of switch section Q which switches in accordance with the control of control section 16 via a closed circuit formed by first diode D1 serially connected to first condenser C1 and a third diode D3 serially connected to second condenser C2.
When the current charges up first and second condensers C1 and C2 of smoothing section 12 by input voltage Vi, input voltage Vi charges up first and second condensers C1 and C2 which are serially connected by input voltage Vi as in the case of applying input voltage Vi to be higher than the charging voltage of first and second condensers C1 and C2 connected by second diode D2. At this time, since the capacitance is reduced due to serially-connected first and second condensers C1 and C2, the charging current charging up first and second condensers C1 and C2 by input voltage Vi has a wide conduction angle to reduce the harmonic current. For this reason, the power factor of the converter which is the electric power supplying apparatus is improved.
However, the smoothing converter according to the conventional technique is charged by applying the high voltage to the first and second condensers which are the bulk condenser employed to the input stage. Consequently, the high voltage is applied to the switch to necessarily require a switch which can sustain the high voltage. Otherwise, a frequency modulating function may be applied to the switch for decreasing the voltage applied to the condenser, which, however, involves a problem of lowering the power factor of the converter because an operating frequency is changed within a wide range. Furthermore, although the foregoing smoothing converter is of the low-priced system, it has a problem that its power factor is not significantly improved to be enough to satisfy the class D standard of IEC-1000-3-2 which is the international standard with respect to the harmonic current.
FIG. 2A is a circuit diagram showing a two stage PFC converter according to the conventional technique, and FIG. 2B is a circuit diagram showing a single phase PFC converter according to the conventional technique.
The two PFC converter circuit according to the conventional technique includes a bridge diode 20 for rectifying an input voltage Vi, and a booster section 22 which is the PFC for switching a second switch section Q2 to improve the power factor of the output of bridge diode 20. Additionally, a driving section 22a senses a voltage applied to a bulk condenser C to switch second switch section Q2 of booster section 22. A transformer T excites the output of booster section 22 from the primary coil side to the secondary coil side, and a switch section Q is switched for pulsewidth-modulating the output of booster section 22 by the control of control section 26. Also, an output section 24 rectifies and smooths the output of the secondary side of transformer T, and control section 26 controls switch section Q by the feedback of an output voltage Vo of output section 24.
The two stage PFC converter circuit according to the conventional technique constructed as above is separately equipped with booster section 22 which is a converter for improving the power factor at the preceding stage of a DC--DC converter for controlling the output voltage. Here, the two stage PFC converter circuit is operated by the two stage of booster section 22 and DC--DC converter such that second switch section Q2 of booster section 22 is switched by means of driving section 22a which is operated separately from control section 26 for making the voltage applied to bulk condenser C constant to the variation of input voltage Vi in the input stage of the converter. By this operation, the harmonic current is decreased in the current supplied to coil L of booster section 22 to improve the power factor.
In more detail, input voltage Vi is rectified in bridge diode 20, and second switch section Q2 is switched to improve the power factor of the rectified output voltage of bridge rectifier 20 in such a manner that the voltage applied to bulk condenser C and that applied to coil L are sensed and compared by driving section 22a which is the PFC control section to apply a stable voltage to bulk condenser C.
The output from switched booster section 22 is to supply the voltage with decreased harmonic current to bulk condenser C, and the voltage charging up bulk condenser C is switched to pulsewidth-modulate switch section Q by the control of control section 26 to be excited from the primary coil side to secondary coil side of transformer T. Then, it is rectified and smoothed in output section 24 to be generated as output voltage Vo.
Also, for securing the stable output of output voltage Vo, output voltage Vo of output section 24 is feedback for switching switch section Q under the control of control section 26 to be controlled.
Booster section 22 is operated to decrease the harmonic wave in input voltage Vi applied from the DC input via coil L since input voltage Vi is switched by second switch section Q2 which becomes an active element being a transistor by driving section 22a to sense the voltage applied to bulk condenser C.
The converter of PFC system further furnished with booster section 22 for improving the power factor by effectively decreasing the harmonic wave as described above utilizes a specified controlling IC to be independently controlled to highly improve the power factor. In addition, the input power source of 110 volts and 220 volts can be utilized without shifting the input power source to be used for a wide input power source.
However, the two stage PFC system independently controlled for improving the power factor separately requires the IC especially for the PFC to complicate the circuit and increase the manufacturing cost.
Referring to FIG. 20 the single stage PFC converter circuit according to the conventional technique is formed by a bridge diode 20 for rectifying an input voltage Vi, and a power factor improving section 23 formed of a coil L, a diode D and a bulk condenser C for improving the power factor of the output of bridge diode 20. A transformer T excites the output of power factor improving section 23 from the primary coil side to the secondary coil side being the output part, and a switch section Q switches the voltage applied to transformer T. Also, an output section 24 rectifies and smooths the secondary side output of transformer T, and a control section 26 to controls switch section Q by sensing to feedback output voltage Vo of output section 24.
The single stage PFC converter circuit according to the conventional technique formed as above has the system of integrating the power factor improving converter and DC--DC converter by a system differently from the double stage PFC converter circuit as shown in FIG. 2A which is separately furnished with booster section 22 being the converter for improving the power factor at the preceding stage of the DC--DC converter for controlling the output voltage.
In other words, the harmonic current of input voltage Vi having passed through bridge diode 20 becomes decreased at coil L, and output of coil L with the decreased harmonic current is supplied to bulk condenser C and switch section Q via respective diodes D, thereby improving the power factor.
Consequently, in the single stage PFC converter circuit according to the conventional technique operated as above, the power factor improving control IC is deleted so as to simultaneously perform the power factor improvement and output voltage control by means the single converter. Furthermore, the number of parts newly added for the power factor improvement is small to allow for realizing the small size and incurring low price.
The harmonic current can be decreased in the single stage PFC converter formed in the low-priced system. But, in the actual application, the great current is supplied to the single switch provided to the preceding stage of the transformer since the booster current and DC--DC converter current operated in the disc continuous conduction mode (DCM) are simultaneously transmitted. For this reason, when the converter is operated at 100 W or higher, at least two switches should be employed to minimize the influence caused by the great current. As the result, in the converter operated at 100 W or higher, the construction consisting of the single stage control and single switch has a problem that the capacitance of the bulk condenser is set to a predetermined value to make the voltage charging up the condenser irregular in conformity with the load variation and input voltage variation, so that the voltage applied to the transformer section becomes unstable to decrease the power factor of the converter.